Various methods and apparatus are known for purifying solvents, particularly water. One such method utilizes the principle of reverse osmosis to reduce or eliminate the quantity of dissolved solids in a liquid. According to the reverse osmosis principle, a semipermeable membrane is used to separate the solvent from the dissolved solids. For example, in purifying water, a membrane is selected that exhibits greater permeability to water than the dissolved solids carried by the water. Raw feed water is applied to the membrane at a pressure generally greater than the osmotic pressure of the water. Under pressure, water passes through the membrane leaving behind the dissolved solids. The liquid passing through the membrane is generally termed "permeate" whereas the liquid remaining on the input side of the membrane is generally termed "concentrate" and is usually discarded to drain.
Since the concentration of solutes increases on the concentrate side of the membrane during the reverse osmosis process, precipitation of one or more of the dissolved solids can occur. This precipitation can cause plugging of the membrane thus lowering the efficiency of the process. In some systems, plugging or compaction of the membrane is compensated for by increasing the pressure of feed water. In other systems, the feed water is fed at a relatively high flow rate to cause turbulence in the vicinity of the membrane. Those employing this arrangement believe that the turbulence prevents the solids from adhering to the membrane. With this method however, a rather large quantity of feed water is discharged as concentrate.
In some systems, the membrane is subject to a continuous pressure by the feed water even during periods of non-use. It is believed that the application of continuous pressure to the membrane reduces its useful life.